(i) Field of the Invention
The present invention relates to a process for producing a polyisoimide.
(ii) Description of the Related Art
It is well known that a polyisoimide can be manufactured by the action of a dehydrating agent on a polyamic acid. For example, in Japanese Patent Application Laid-open Nos. 60-170683, 61-254547, 63-118329, 63-118330 and 63-223746, Japanese Patent Disclosure Nos. 61-500851 and 61-500997, and U.S. Pat. Nos. 3,271,366, 3,282,898 and 4,699,803, there are used, as dehydrating agents, an N,N'-dihydrocarbyl-substituted carbodiimide such as dicyclohexylcarbodiimide (hereinafter referred to as "DCC"), a halogenated lower fatty acid anhydride such as trifluoroacetic acid anhydride, and a lower fatty acid halide such as thionyl chloride or acetyl chloride.
However, in the case that N,N'-dihydrocarbyl-substituted carbodiimide is used, a urea compound which is insoluble in the solvent used is formed as a by-product by a dehydration reaction. Therefore, it is necessary to separate the urea compound which is the by-product, which complicates the manufacturing process Furthermore, the halogenated lower fatty acid anhydride and the lower fatty acid halide as well as the above-mentioned N,N'-dihydrocarbyl-substituted carbodiimide are very poisonous, and hence much attention must be paid to the handling of these dehydrating agents.